The present invention relates to a hydraulic system having an axial piston pump as a control pump.
Hydraulic systems with axial piston control pumps are being used increasingly in agricultural tractors and construction machines. In such systems, fluid under pressure is supplied by axial piston pumps which, depending on their principle of construction--oblique disk or oblique axis--make it possible to vary delivery volume of hydraulic fluid. For energy considerations, systems which are subject to load control are preferred, i.e., systems in which the specific pump delivery stream and pressure are adjusted as a function of the pressure and volume required by the load.
Axial piston pumps are typically sensitive to excessively high vacuums in their suction line. Therefore, when an axial piston control pump is employed in a vehicle, a feed pump is preferably used to convey the hydraulic fluid from a storage container into the suction line of the axial piston control pump, and thereby continuously assure sufficient fluid pressure. The feed pump utilized is, as a rule, a pump with a constant volumetric flow, such as a gear pump, and must therefor be dimensioned so that it delivers the maximum possible quantity to the control pump. This means, however, that under operating conditions in which the hydraulic circuit supplied by the control pump requires little or no pressure fluid, the amount fed by the feed pump must be diverted into the storage container. Although it is possible to use a part of this excess quantity of fluid for lubricating and cooling purposes, such as for lubricating the transmission of a vehicle, it is uneconomical to use the pump in this manner.
If one considers a load/time diagram, of, for example, agricultural tractors, it is apparent that maximum delivery of the control pump is required very rarely. Typical load/time diagrams, in fact, indicate that maximum delivery is required less than 1% of the total operating time. Moreover, if the supply of lubricating oil to the transmission depends on the feed circuit which is supplied by the feed pump, much energy can be saved through the use of smaller feed pumps.
It would be possible to adjust the feed pump such that it satisfies, in addition to the other demands made on it (e.g. lubrication), only part of the quantity required by the control pump. If the control pump however requires volumetric flows which exceed this partial requirement then the amount of hydraulic fluid in excess thereof must be drawn directly out of the storage container via an additional line. This manner of operation is, however, permissible for axial piston pumps only if certain operating conditions are satisfied. For instance, the additional line must have a suitable cross-section, and hydraulic fluid which is sufficiently warm must be used.
However, on a cold start, particularly at low outdoor temperatures, the resultant higher viscosity of the hydraulic fluid (which may be, for instance, oil) causes large pressure drops to occur already at low flow velocities in the suction line. This leads to the possibility of occurrence of, high vacuums on the suction side of the control pump, which can damage the control pump due to cavitation phenomena effects.
The object of the present invention, therefore, is to provide a hydraulic system having a feed pump and an axial piston control pump, in which the feed pump is designed optimally from an energy standpoint with regard to the load-requirement profile for the hydraulic system, while nevertheless avoiding impermissible operating conditions for the control pump.